Add support for holdover

[jclark]

With the BG7TBL GNSSDO-CM55 (see #151) we can potentially support holdover in a useful way.

If the GNSS loses its lock, then we will get time messages that say the time is invalid, but we will continue to get a pulse.

We need to pay attention to these invalid time messages and recognize them as indicating we have entered a holdover state.

We should then update the PTP grandmaster to change to the holdover ClockClass.

I haven't yet tested what happens when it regains lock after an extended period of holdover.

This could also be potentially made to work with the Leo Bodnar LBE-1420 and LBE-1421 (but see #87).